Long Term Evolution (LTE), commonly marketed as 4G LTE, is a standard for wireless communication of high-speed data for mobile phones and data terminals. LTE is based on Global System for Mobile Communications (GSM) and Universal Mobile Telecommunication System (UMTS) technologies that provides higher data rate, lower latency and improved system capacity. In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, referred as evolved Node-Bs (eNBs), communicating with a plurality of mobile stations, referred as user equipments (UEs).
The LTE standard is developed by 3GPP (3rd Generation Partnership Project) and is specified in its Release 8 document series, with enhancement described in later releases. LTE is the natural upgrade path for carriers with both GSM/UMTS networks and CDMA2000 networks. Along with LTE introduction, 3GPP also standardized Single Radio Voice Call Continuity (SRVCC) from Release 9 specifications to provide seamless continuity when a UE handovers from LTE coverage (E-UTRAN) to UMTS/GSM coverage (UTRAN/GERAN). With SRVCC, a call anchored in IP multimedia subsystem (IMS) core can continue to be served by UMTS and GSM networks outside of LTE coverage area.
3GPP Release 9 introduced the basic SRVCC scenario, which is to transfer a single call from IMS domain to circuit-switch (CS) domain. Further in later releases, emergency call, call in different states (e.g., hold, conference), alerting call, and video call are also supported to be transferred from IMS to CS. However, there are still some scenarios that are not defined clearly when multiple IMS ongoing sessions are established. Due to the lack of consistent agreement in 3GPP specification, the misbehaving UE implementation will result in unexpected call drop and/or un-synchronous call state after the UE camps to a new radio access network or new radio access technology.
To solve the problem, a scheme is disclosed in the present invention to manage call continuity in a network environment including a circuit-switched network and an IP multimedia subsystem (IMS) network during different circumstances.